This was my HR plot from my 945 (with HRM-Tri strap) last week. No idea why the HR suddenly jumps after 5 minutes, it was definitely under-reading in those first 5 minutes as that run was a relatively steady state effort.
This probably happens every one in 50 runs at a guess.
This is my take on it too. I just need a guide, not an exact HR figure. Gave my HR strap away ages ago.